Non-uniformities of optical paths such as those caused by heterogeneity of the ambient environment—for example, air turbulence due to air movement or non-uniformities of the refractive index—continue to present a practical problem for operation of an optical autofocus (AF) system (unless such a system is completely sealed off from the environment, which is costly and not always practically feasible for operation). The presence of air turbulence, for example, manifests in changes in the phase acquired by light propagating through the fringe-projection system of an optical autofocus system on its way to the detector, and, therefore, substantially un-quantifiably contributes to the error in the measurement of a target surface profile. As the degree of accuracy and precision of the operation of the AF systems continues to improve, there remains an unmet need in an operational modality that enables to identify the air-turbulence related measurement error, controllably and with a known degree of certainty.